I wish to investigate the differences in grinding of slag over clinker. We have a 100tph OPC cement ball mill. Could this mill grind slag? What modification would be needed? Is it always necessary to supply hot air when grinding slag? What sort percentage drop in production should be expected?

Yes, your ball mill can grind slag. If it were to be permanently converted to slag grinding then a change to a finer ball grading would be necessary. Slag is normally ground finer than cement and has a smaller grain size to begin with. Certainly there is a limit to the drying capacity of a mill based on the heat generated in the mill alone. To determine whether you need to add hot air you need to develop a heat balance of the mill and test various scenarios with different moisture contents of slag and feed rates. Slag is much harder than cement clinker and therefore you could expect as much as a 50 per cent reduction in output from the mill.

Do you think that pre-dedusting in a raw mill circuit by means of cyclone with a separation efficiency of 91-93 will have negative influence on the raw mill bag filter or ESP in regard to the fact that after pre-dedusting only rather fine grain and rather low quantity of dust will enter the filter? Could it become difficult to clean the bags due to fine dust getting deeper into the pores of the filterbag and could the fine dust have negative influence on the ESP?

I cannot foresee that reduces the load of dust entering a filter will have any detrimental effect on its performance. Certainly no problem with an ESP. Your point regarding fine dust blinding the cloth of fabric filters may be valid, but I would be surprised.

We will appreciate your support clarifying the following points:
1-False air effect upon the (increase/decrease) of dust load in a closed system of air swept type ball mill (between the mill outlet duct-seperator-cyclones-filter-stack). 2 - Also, will the dust resistivity increase/decrease?

In an air-swept mill false air is usually introduced at the bottom of the ascension pipe in order to ensure there is sufficient volume, density and velocity of air to lift the ground material up the ascension pipe and through the separator. The amount of dust being carried up the ascension pipe is the same, however the volume of air is greater therefore the overall effect is a dilution of the dust load. In principle the false air will cause a reduction of the temperature and this will reduce the resistivity of the dust improving the electrostatic precipitation.

I would like to know how to estimate the quantity of grinding media of each size required for a ball mill using the data mill size, number of chambers and type of material to be ground. Kindly explain me what formula is to be used.

The total quantity of media required for each chamber is determined by calculating the internal volume of the chambers. A rule of thumb is to change the first chamber to 30 per cent of this volume and the second chamber to 27 per cent. You can use 4.6t/m₃ as the density of grinding media. As for the individual ball size gradings this has to be determined by conducting a series of axial tests in the mill while you are building up the media charge.

Are there any recognised ways to increase raw mill production?

The first stage is to maximise the output with the existing mill. You should conduct axial tests and recirculating load tests to make sure that the ball charge and lining are optimal. The next stage might be to conduct trials with grinding aids to try to boost the output of the mill by up to 15 per cent. Beyond that you will need to consider modifications to the milling circuit by adding further equipment. There are two options to do that:
1. Add a pregrinding facility such as a hammer mill or roll press, or
2. Add a regrind mill. Adding further equipment will be significantly more expensive than introducing grinding aids but has the potential to boost the output to higher levels.